1. Field of the Invention
The present invention relates to the production of porous carbon structures and particularly to the manufacture of articles comprised of binderless, single phase, bulk carbon having a fibrillar microstructure. More specifically, this invention is directed to fibrillar carbon fuel cell electrode substrates. Accordingly, the general objects of the present invention are to provide novel and improved methods and articles of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to the production of carbon or graphite "paper." Such carbon "paper" has been employed as electrode support plates and could be substituted for present porous sintered nickle electrolyte reservoir plates in fuel cells. For a fuel cell application, carbon "paper" must, in addition to being cost effective, meet rather stringent chemical, physical and electrical property requirements. Thus, by way of example, material suitable for use in a fuel cell as an electrode support plate or an electrolyte reservoir plate must be characterized by a high resistance to oxidation, availability in thickness in the range of 15-20 mils and be susceptible to handling; i.e., not fragile. Also, such material must have a high degree of porosity and have a lateral resistivity which lies within a narrow predetermined range. As an example of utilization of a porous sheet material in the fuel cell technology, such material may be employed as electrolyte reservoir plates. An electrolyte reservoir plate in a fuel cell will contain excess KOH solution within its pores, which have an average pore size in the range of 3-8.mu., and will serve to assure that the maximum amount of the electrolyte communicates with the cell. The electrolyte reservoir plate is also the media through which the product water produced in the fuel cell is removed. In addition to being lightweight, having high pore volume and KOH compatability, the electrolyte reservoir plate material must be easily wet and contain a narrow pore size distribution to facilitate delivery of electrolyte from its structure to the cell matrix on demand. In an acid electrolyte fuel cell, carbon "paper," having an average pore size in the range of 15-30.mu., presently functions as support plates to hold the active catalyst of the fuel cells.
The conventional prior art manner of forming porous carbon structures suitable for use as fuel cell catalyst support plates consists of forming chopped carbon fibers into paper-like structures and subsequently bonding the fibers together by chemical vapor deposition of a carbon binder phase. Alternately, porous carbon "paper," may be fabricated by bonding the chopped carbon fibers together with a polymer resin which is subsequently pyrolyzed to form a carbon binder. These prior art techniques are inherently slow and costly. A primary contributing factor to the high cost of the prior art techniques resides in the use of exceedingly expensive graphite or carbon fibers as the starting material. The use of graphite or carbon fibers as the starting material dictates that the production of the porous carbon "paper" include two graphitization steps; i.e., graphitization to produce the starting fiber and then a further pyrolyzation step to convert the binder material to carbon.
It has recently been discovered that binderless, single phase bulk carbon or graphite articles having a fibrillar microstructure can be produced through stabilizing certain polymer fibers by a heating step, which introduces a selected amount by weight of oxygen into the polymer, and thereafter subjecting short lengths of the stabilized fiber to a temperature and pressure sufficient to cause heat distortion flow and concomitant self-bonding of contiguous fibers. Thereafter the bonded structure is pyrolyzed by heating in a nonoxidizing atmosphere. A complete discussion of the above briefly described technique, as well as detailed discussion of the previous state of the art, may be found in copending U.S. patent application Ser. No. 661,211 of George K. Layden, Romeo G. Bourdeau and Roscoe A. Pike; application Ser. No. 661,211 being a continuation of application Ser. No. 481,438, now abandoned, filed June 20, 1974. The disclosure of application Ser. No. 661,211 is incorporated herein by reference. The technique of referenced application Ser. No. 661,211, if adapted to the production of porous carbon "paper" suitable for fuel cell applications, would substantially reduce the cost of such "paper" through elimination of the need to use expensive carbon fibers and through elimination of the use of a polymer resin binder or a chemical vapor deposited carbon binder phase.